1. Field of the Invention
The invention relates to methods of controlled remelting of or laser metal forming on the surface of articles.
2. Brief Description of the Related Art
In the last years laser metal forming (LMF) has been introduced in industrial manufacturing. Laser metal forming is a process where a high power laser locally melts a focused stream of metal powder or a metal wire 20 onto a substrate. In this way material can be added to the underlying part. The method is suitable for controlled material build-up and the laser generated parts are characterized by a dense microstructure which is usually free of pores.
Laser metal forming has been recently used for commercial manufacturing of superalloys due to its attractive potential for repair of locally damaged or worn parts. Indeed, it is possible to selectively add material at desired locations and to re-establish the full functionality of a component. It is clear that laser repair technology is particularly attractive for the refurbishment of expensive parts that are affected by local damage or local mechanical wear. Turbine blades and vanes are typical examples.
However, the process is complicated when single-crystal components have to be refurbished. Single crystal blades and vanes can be found in the most heavily loaded rows of modern gas turbines (first or high pressure row). Their mechanical integrity relies on the particular properties due to single-crystal microstructure and the absence of grain boundaries. Reconditioning of such components is only feasible if the single crystal microstructure can be maintained during the repair process.
During laser metal forming substrate material is locally molten and powder (or wire 20) is injected into the melt pool with a suitable powder (or wire 20) feeder mechanism. After a certain interaction time (which is determined by the laser spot size and the relative movement between laser and substrate) the molten material resolidifies leading to material build-up on the substrate.
However, during the solidification of the molten material new grains may form in the melt pool due to constitutional undercooling of the liquid melt. The growth of these newly formed grains leads to undesired build-up of equiaxed material, i.e. material that is oriented in a random manner. As the thermomechanical properties of superalloys greatly depend on the crystallographic orientation and as SX crystal components rely on the benefit of preferable orientation it is obvious that the consequence of equiaxed growth is a serious degradation of the mechanical properties. Moreover, superalloys which do not contain grain boundary stabilizing elements exhibit excessive creep when unwanted grain boundaries are formed. For epitaxial laser metal forming it is therefore crucial to ensure a completely SX microstructure of a part by avoiding the so-called columnar to equiaxed transition (CET).
One strategy for ensuring epitaxial growth, i.e. growth with orientation matched to the substrate and without formation of new grains, is to use special process conditions. Laser parameters have to be adjusted in a manner that a specific ratio between temperature gradient G and the solidification speed Vs is maintained. Both quantities depend not only on laser parameters such as power, power density, advance speed but also on the properties of the substrate and powder (or wire 20) material.
Those skilled in the art of laser metal forming are also aware that the onset of (marangoni) convection in the melt pool is one of the main reasons for the undesired CET. Initiation of convection processes in the melt pool leads to fragmentation of the fragile dendrites that form during the solidification of the molten material. By the effect of convective transport dendrite fragments are distributed all over the melt pool where they acts as nucleation sites and promote the formation of equiaxed material.
Unfortunately melt pool convection is also affected by other process parameters like mass feed rate, protection gas stream, injection angle. In addition, marangoni convection is not readily detectable without melt pool monitoring.
So far, several patents have been issued for the laser metal forming process. The basic principle is described in EP-A1-0 558 870, DE-C1-199 49 972, U.S. Pat. No. 5,873,960, U.S. Pat. No. 5,622,638 or U.S. Pat. No. 4,323,756.
The application of epitaxial material build-up for protective coatings is covered by U.S. Pat. No. 6,277,500, applications for generation or refurbishment of single crystal components are described in U.S. Pat. No. 6,024,792, EP-A1-0 740 977, WO95/35396 or U.S. Pat. No. 5,914,059. Except U.S. Pat. No. 6,024,792 none of these patents mentions the significance of the G, Vs parameters in order to obtain the desired single crystal microstructure. U.S. Pat. No. 6,024,792 states that the laser power has to be set in a way to obtain adequate values for G and Vs, however, does not suggest a method for automatic laser power control or for avoiding melt pool convections.
Another patent application, WO95/06540 suggests the use of a pyrometer for interactive laser welding of super alloy articles measuring the substrate preheating temperature.
The collection of optical signals from the melt pool is also depicted in U.S. Pat. No. 6,122,564. In this patent, an optical monitoring system is connected to a feedback controller in order to adjust the material deposition rate depending on the indicated height of previously deposited material.
In U.S. Pat. No. 6,311,099 an apparatus for regulating laser welding parameters is suggested that uses optical signals from the interaction zone. In this patent the optical signal is generated by near infrared radiation originating from the weld pool. The radiation is detected by a CCD camera and processed in order to obtain information about the physical dimensions of the melt pool.